After how many weeks I can separate baby bird from its mother?

u can hand feed at three weeks

Personally, I would advise that since you have no idea about what you are going to "try", doing and no experience with hand feeding any living thing, other than yourself, that you get taught how to do it, after watching someone else do it real time from start to the birds getting off formula to solid goods.

So your are NOT going to be doing it with this clutch, period. If you work outside the home, and do not have an eye for detail, it is not for you until you have a solid month 24/7 to devote to feeding, making FRESH, formula at the right consistency, temperature for each phase of their feedings. Cleaning them ip afterwards, keeping weight charts to make sure they are reaching their goals.  Most importantly, if you do not know the signs of a bird with sour croup, and how to deal with it, you will be killing these baby birds. 

Trust me this is NOT, a YouTube or 5 page quick guide, and it's all good. You need hands on experience with someone that's doing it for years. Hand feeding baby birds can be easy once you know what you are doing. 

But your main question tells me you don't even have the basics onboard to take the babies to just hold them, let alone feed them, two fill days without issues. 

That sounds cruel. 

If you know what you are doing, you can start hand feeding as soon as they hatch.

It is demanding, time consuming, and hard work.

You need to maintain temperature, hygiene, and know how to mix formula at correct temperature, quantity and frequency.

Waiting until they are partially feathered is much easier.